The present invention relates to a gas turbine having a closed-circuit cooling system for one or more nozzle stages and, more particularly, to a gas turbine having inserts for impingement-cooling of the nozzle vane walls and which inserts are sectional to facilitate installation into the nozzle vane cavities.
In advanced gas turbines, nozzle stages are often provided with a closed-circuit cooling system for cooling the nozzle vanes exposed to the hot gas path. For example, each nozzle vane may include a plurality of cavities extending between the outer and inner nozzle bands. Impingement-cooling inserts are provided in one or more cavities and a cooling medium such as steam is passed into the insert and through apertures in the side walls of the insert for impingement-cooling the adjacent wall portions of the nozzle vane. An example of a closed-circuit steam-cooled nozzle for a gas turbine is disclosed in U.S. Pat. No. 5,743,708, of common assignee herewith, the disclosure of which is incorporated herein by reference.
Typically, the nozzle insert is a unitary body provided by an insert supplier and nominally sized for reception within the cavity of the nozzle vane. It will be appreciated that the insert is constructed and arranged so that when it is inserted into the vane cavity, an impingement gap is defined between the interior wall of the nozzle and the wall of the insert. However, because of manufacturing tolerances involved with the nozzle cavity and the insert per se, as well as the need to be able to dispose the insert endwise into the nozzle cavity, variations from the designed impingement gap along the length of the insert and nozzle vane wall frequently occur. A variation in the impingement gap can, in turn, cause a significant change in the heat transfer between the nozzle vane walls and the cooling medium. For example, it has been found that a 0.010 inch variation in the gap from a nominal dimension can result in an approximate 13% reduction in heat transfer coefficient. Also, this percentage increases exponentially with further impingement gap variation. Further, installation of a unitary insert into the nozzle vane cavity is somewhat difficult, oftentimes requiring a custom fit. There is also a potential for low-cycle fatigue as a result of the variation in heat transfer coefficient caused by the varying impingement gap.
To facilitate design, manufacture and installation of an airfoil impingement insert in steam cooled nozzles, a divided insert is proposed, that may be made in two halves so as to facilitate manufacture and installation. Impingement inserts are typically made of an alloy, such as Inco 625 and have thin wall sections. When an insert is made as a divided structure, with two leg sections, there is a need for a separator structure to maintain the spacing of the individual sections for achieving and maintaining a target impingement gap and for mechanical support. Thus, the invention provides a support and separator bar or rod to provide the support and distance separation required to meet life and operational needs.
A split insert which resulted from a parallel development is disclosed in commonly assigned U.S. Pat. No. 6,450,759, the disclosure of which is incorporated herein by reference. In that adaptation, spreader plates are secured to the inner wall portions of the insert sections to maintain the insert sections spaced from one another. In contrast to the spreader plates of the ""759 split insert, the present invention provides an insert having two leg sections with support bars or rods to maintain the insert sections spaced from one another. The rod-type support has the significant advantages of reduced area, greater strength, and easy and secure attachment.
Thus, the invention is embodied in an insert for a cavity of a nozzle vane of a gas turbine for impingement-cooling of the walls of the vane, comprising: a pair of elongated hollow leg sections disposable in side-by-side relation to one another within the cavity, said leg sections having a plurality of apertures through oppositely directed outer walls thereof, inner wall portions of the leg sections being spaced from one another; and at least one support rod extending between said inner wall portions of said leg sections for maintaining said inner wall portions of said leg sections spaced from one another.
The invention is also embodied in an insert disposed in a cavity of a nozzle vane of a gas turbine for impingement-cooling walls of the vane, wherein the assembly comprises: a pair of elongated hollow insert leg sections disposable in side-by-side relation to one another within the cavity, said leg sections having a plurality of apertures through oppositely directed outer walls thereof, inner wall portions of the leg sections being spaced from one another; and at least one support rod disposed to extend between said leg sections for maintaining said inner wall portions of said leg sections spaced from one another, first end portions of said leg sections having inner wall surfaces disposed for engagement with one another to facilitate securement of said leg sections to one another. The invention may also be embodied in a method of installing a cooling medium insert into a cavity of a nozzle vane for a gas turbine wherein the insert includes a pair of elongated hollow leg sections, each having an outer wall portion with a plurality of apertures therethrough, comprising: (a) inserting the leg sections into the vane cavity for disposition therein in side-by-side relation to one another, with the outer wall portions thereof in opposed facing relation to side wall portions of said vane; and (b) subsequent to step (a), and while the leg sections remain in the vane cavity, inserting a support rod to extend between spaced inner wall portions of said leg sections to support and maintain said leg sections in spaced relation.